What is the best smart pointer return type for a factory function?

Question

With respect to smart pointers and new C++11/14 features, I am wondering what the best-practice return values and function parameter types would be for classes that have the

Answer 1

What would the best return type be for the factory function?

unique_ptr would be best. It prevents accidental leaks, and the user can release ownership from the pointer, or transfer ownership to a shared_ptr (which has a constructor for that very purpose), if they want to use a different ownership scheme.

What is the best parameter type for the utility function?

A reference, unless the program flow is so convoluted that the object might be destroyed during the function call, in which case shared_ptr or weak_ptr. (In either case, it can refer to a base class, and add const qualifiers, if you want that.)

What is the best parameter type for the function that keeps a reference to the object?

shared_ptr or unique_ptr, if you want it to take responsibility for the object's lifetime and not otherwise worry about it. A raw pointer or reference, if you can (simply and reliably) arrange for the object to outlive everything that uses it.